Abstract : Solving an Image Processing problem involves three main categories of experts: experts from the domain of application (biologists, geographers... ), Image Processing experts and designer-programmers. Domain experts are requested to express clearly and exhaustively the purpose of their application. Image Processing experts have to propose somne kind of answer in terms of Image Processing plans adapted to the specifie features of the request and the nature of images. Designer-programmers have to provide a framework for an efficient implementation. We are interested in setting up a Knowledge-Based workbench directed toward the cooperation between these experts and the system. This approach is motivated by the development of real-world applications, while being concerned in generic principles and knowledge acquisition and integration concerns. It hinges on the supervision of a library of Image Processing operators. Solving a specifie application is a planning process which implies selecting the suitable operators, linking them and tuning their parameters. An application is thus represented as an Image Processing plan. After giving the specifications of our system, which belongs to the more general class of Knowledge-Based systems for program supervision, our software architecture favoring cooperation as well as knowledge integration will be presented. This ref1exive architecture is based on the TASK/METHOD/TOOL paradigm and provides conceptual models of applications that are actually computational. Several Image Processing plans dealing with biomedical (cytological / histological) images have already been built within our workbench, which constitutes a first convincing validation of our approach.